/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 * 
 * 
 * Modified by Vladimir Blagojevic to include lock amortized eviction. 
 * For more details see http://www.cse.ohio-state.edu/hpcs/WWW/HTML/publications/papers/TR-09-1.pdf
 * https://jira.jboss.org/jira/browse/ISPN-299 
 * 
 */

package org.infinispan.util.concurrent;

import java.io.IOException;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Collections;
import java.util.ConcurrentModificationException;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.locks.ReentrantLock;

/**
 * A hash table supporting full concurrency of retrievals and adjustable expected concurrency for
 * updates. This class obeys the same functional specification as {@link java.util.Hashtable}, and
 * includes versions of methods corresponding to each method of <tt>Hashtable</tt>. However, even
 * though all operations are thread-safe, retrieval operations do <em>not</em> entail locking, and
 * there is <em>not</em> any support for locking the entire table in a way that prevents all access.
 * This class is fully interoperable with <tt>Hashtable</tt> in programs that rely on its thread
 * safety but not on its synchronization details.
 * 
 * <p>
 * Retrieval operations (including <tt>get</tt>) generally do not block, so may overlap with update
 * operations (including <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results of the
 * most recently <em>completed</em> update operations holding upon their onset. For aggregate
 * operations such as <tt>putAll</tt> and <tt>clear</tt>, concurrent retrievals may reflect
 * insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements
 * reflecting the state of the hash table at some point at or since the creation of the
 * iterator/enumeration. They do <em>not</em> throw {@link ConcurrentModificationException}.
 * However, iterators are designed to be used by only one thread at a time.
 * 
 * <p>
 * The allowed concurrency among update operations is guided by the optional
 * <tt>concurrencyLevel</tt> constructor argument (default <tt>16</tt>), which is used as a hint for
 * internal sizing. The table is internally partitioned to try to permit the indicated number of
 * concurrent updates without contention. Because placement in hash tables is essentially random,
 * the actual concurrency will vary. Ideally, you should choose a value to accommodate as many
 * threads as will ever concurrently modify the table. Using a significantly higher value than you
 * need can waste space and time, and a significantly lower value can lead to thread contention. But
 * overestimates and underestimates within an order of magnitude do not usually have much noticeable
 * impact. A value of one is appropriate when it is known that only one thread will modify and all
 * others will only read. Also, resizing this or any other kind of hash table is a relatively slow
 * operation, so, when possible, it is a good idea to provide estimates of expected table sizes in
 * constructors.
 * 
 * <p>
 * This class and its views and iterators implement all of the <em>optional</em> methods of the
 * {@link Map} and {@link Iterator} interfaces.
 * 
 * <p>
 * Like {@link Hashtable} but unlike {@link HashMap}, this class does <em>not</em> allow
 * <tt>null</tt> to be used as a key or value.
 * 
 * <p>
 * This class is a member of the <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 * 
 * @since 1.5
 * @author Doug Lea
 * @param <K>
 *            the type of keys maintained by this map
 * @param <V>
 *            the type of mapped values
 */
public class BufferedConcurrentHashMap<K, V> extends AbstractMap<K, V> implements
                ConcurrentMap<K, V>, Serializable {
    private static final long serialVersionUID = 7249069246763182397L;

    /*
     * The basic strategy is to subdivide the table among Segments, each of which itself is a
     * concurrently readable hash table.
     */

    /* ---------------- Constants -------------- */
    /**
     * The default initial capacity for this table, used when not otherwise specified in a
     * constructor.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**
     * The default load factor for this table, used when not otherwise specified in a constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The default concurrency level for this table, used when not otherwise specified in a
     * constructor.
     */
    static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    /**
     * The maximum capacity, used if a higher value is implicitly specified by either of the
     * constructors with arguments. MUST be a power of two <= 1<<30 to ensure that entries are
     * indexable using ints.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The maximum number of segments to allow; used to bound constructor arguments.
     */
    static final int MAX_SEGMENTS = 1 << 16; // slightly conservative

    /**
     * Number of unsynchronized retries in size and containsValue methods before resorting to
     * locking. This is used to avoid unbounded retries if tables undergo continuous modification
     * which would make it impossible to obtain an accurate result.
     */
    static final int RETRIES_BEFORE_LOCK = 2;

    /* ---------------- Fields -------------- */

    /**
     * Mask value for indexing into segments. The upper bits of a key's hash code are used to choose
     * the segment.
     */
    final int segmentMask;

    /**
     * Shift value for indexing within segments.
     */
    final int segmentShift;

    /**
     * The segments, each of which is a specialized hash table
     */
    final Segment<K, V>[] segments;

    transient Set<K> keySet;
    transient Set<Map.Entry<K, V>> entrySet;
    transient Collection<V> values;

    /* ---------------- Small Utilities -------------- */

    /**
     * Applies a supplemental hash function to a given hashCode, which defends against poor quality
     * hash functions. This is critical because ConcurrentHashMap uses power-of-two length hash
     * tables, that otherwise encounter collisions for hashCodes that do not differ in lower or
     * upper bits.
     */
    private static int hash(int h) {
        // Spread bits to regularize both segment and index locations,
        // using variant of single-word Wang/Jenkins hash.
        h += (h << 15) ^ 0xffffcd7d;
        h ^= (h >>> 10);
        h += (h << 3);
        h ^= (h >>> 6);
        h += (h << 2) + (h << 14);
        return h ^ (h >>> 16);
    }

    /**
     * Returns the segment that should be used for key with given hash
     * 
     * @param hash
     *            the hash code for the key
     * @return the segment
     */
    final Segment<K, V> segmentFor(int hash) {
        return segments[(hash >>> segmentShift) & segmentMask];
    }

    /* ---------------- Inner Classes -------------- */

    /**
     * ConcurrentHashMap list entry. Note that this is never exported out as a user-visible
     * Map.Entry.
     * 
     * Because the value field is volatile, not final, it is legal wrt the Java Memory Model for an
     * unsynchronized reader to see null instead of initial value when read via a data race.
     * Although a reordering leading to this is not likely to ever actually occur, the
     * Segment.readValueUnderLock method is used as a backup in case a null (pre-initialized) value
     * is ever seen in an unsynchronized access method.
     */
    static final class HashEntry<K, V> {
        final K key;
        final int hash;
        volatile V value;
        final HashEntry<K, V> next;
        volatile Recency state;

        HashEntry(K key, int hash, HashEntry<K, V> next, V value) {
            this.key = key;
            this.hash = hash;
            this.next = next;
            this.value = value;
            this.state = Recency.HIR_RESIDENT;
        }

        public int hashCode() {
            int result = 17;
            result = (result * 31) + hash;
            result = (result * 31) + key.hashCode();
            return result;
        }

        public boolean equals(Object o) {
            // HashEntry is internal class, never leaks out of CHM, hence slight optimization
            if (this == o)
                return true;
            if (o == null)
                return false;
            HashEntry<?, ?> other = (HashEntry<?, ?>) o;
            return hash == other.hash && key.equals(other.key);
        }

        public void transitionHIRResidentToLIRResident() {
            assert state == Recency.HIR_RESIDENT;
            state = Recency.LIR_RESIDENT;
        }

        public void transitionHIRResidentToHIRNonResident() {
            assert state == Recency.HIR_RESIDENT;
            state = Recency.HIR_NONRESIDENT;
        }

        public void transitionHIRNonResidentToLIRResident() {
            assert state == Recency.HIR_NONRESIDENT;
            state = Recency.LIR_RESIDENT;
        }

        public void transitionLIRResidentToHIRResident() {
            assert state == Recency.LIR_RESIDENT;
            state = Recency.HIR_RESIDENT;
        }

        public Recency recency() {
            return state;
        }

        @SuppressWarnings("unchecked")
        static <K, V> HashEntry<K, V>[] newArray(int i) {
            return new HashEntry[i];
        }
    }

    private enum Recency {
        HIR_RESIDENT, LIR_RESIDENT, HIR_NONRESIDENT
    }

    public enum Eviction {
        NONE {
            //@Override
            public <K, V> EvictionPolicy<K, V> make(Segment<K, V> s, int capacity, float lf) {
                return new NullEvictionPolicy<K, V>();
            }
        },
        LRU {

            //@Override
            public <K, V> EvictionPolicy<K, V> make(Segment<K, V> s, int capacity, float lf) {
                return new LRU<K, V>(s,capacity,lf,capacity*10,lf);
            }
        },
        LIRS {
            //@Override
            public <K, V> EvictionPolicy<K, V> make(Segment<K, V> s, int capacity, float lf) {
                return new LIRS<K,V>(s,capacity,lf,capacity*10,lf);
            }
        };

        abstract <K, V> EvictionPolicy<K, V> make(Segment<K, V> s, int capacity, float lf);
    }
    
    public interface EvictionListener<K, V> {
        void evicted(K key, V value);
    }
    
    static class NullEvictionListener<K,V> implements EvictionListener<K, V>{
        //@Override
        public void evicted(K key, V value) {            
        }        
    }

    interface EvictionPolicy<K, V> {

        public final static int MAX_BATCH_SIZE = 64;

        /**
         * Invokes eviction policy algorithm and returns set of evicted entries.
         * 
         * <p>
         * Set cannot be null but could possibly be an empty set.
         * 
         * @return set of evicted entries.
         */
        Set<HashEntry<K, V>> execute();

        /**
         * Invoked to notify EvictionPolicy implementation that there has been an attempt to access
         * an entry in Segment, however that entry was not present in Segment.
         * 
         * @param e
         *            accessed entry in Segment
         */
        void onEntryMiss(HashEntry<K, V> e);

        /**
         * Invoked to notify EvictionPolicy implementation that an entry in Segment has been
         * accessed. Returns true if batching threshold has been reached, false otherwise.
         * <p>
         * Note that this method is potentially invoked without holding a lock on Segment.
         * 
         * @return true if batching threshold has been reached, false otherwise.
         * 
         * @param e
         *            accessed entry in Segment
         */
        boolean onEntryHit(HashEntry<K, V> e);

        /**
         * Invoked to notify EvictionPolicy implementation that an entry e has been removed from
         * Segment.
         * 
         * @param e
         *            removed entry in Segment
         */
        void onEntryRemove(HashEntry<K, V> e);

        /**
         * Invoked to notify EvictionPolicy implementation that all Segment entries have been
         * cleared.
         * 
         */
        void clear();

        /**
         * Returns type of eviction algorithm (strategy).
         * 
         * @return type of eviction algorithm
         */
        Eviction strategy();

        /**
         * Returns true if batching threshold has expired, false otherwise.
         * <p>
         * Note that this method is potentially invoked without holding a lock on Segment.
         * 
         * @return true if batching threshold has expired, false otherwise.
         */
        boolean thresholdExpired();
    }

    static class NullEvictionPolicy<K, V> implements EvictionPolicy<K, V> {

        //@Override
        public void clear() {
        }

        //@Override
        public Set<HashEntry<K, V>> execute() {
            return Collections.emptySet();
        }

        //@Override
        public boolean onEntryHit(HashEntry<K, V> e) {
            return false;
        }

        //@Override
        public void onEntryMiss(HashEntry<K, V> e) {
        }

        //@Override
        public void onEntryRemove(HashEntry<K, V> e) {
        }

        //@Override
        public boolean thresholdExpired() {
            return false;
        }

        //@Override
        public Eviction strategy() {
            return Eviction.NONE;
        }
    }

    static final class LRU<K, V> implements EvictionPolicy<K, V> {
        private final ConcurrentLinkedQueue<HashEntry<K, V>> accessQueue;
        private final Segment<K,V> segment;
        private final LinkedList<HashEntry<K, V>> lruQueue;
        private final int maxBatchQueueSize;
        private final int trimDownSize;
        private final float batchThresholdFactor;

        public LRU(Segment<K,V> s, int capacity, float lf, int maxBatchSize, float batchThresholdFactor) {
            this.segment = s;
            this.trimDownSize = (int) (capacity * lf);
            this.maxBatchQueueSize = maxBatchSize > MAX_BATCH_SIZE ? MAX_BATCH_SIZE : maxBatchSize;
            this.batchThresholdFactor = batchThresholdFactor;
            this.accessQueue = new ConcurrentLinkedQueue<HashEntry<K, V>>();
            this.lruQueue = new LinkedList<HashEntry<K, V>>();
        }

        //@Override
        public Set<HashEntry<K, V>> execute() {
            Set<HashEntry<K, V>> evicted = Collections.emptySet();
            if (isOverflow()) {
                evicted = new HashSet<HashEntry<K, V>>();
            }
            try {
                for (HashEntry<K, V> e : accessQueue) {
                    if (lruQueue.remove(e)) {
                        lruQueue.addFirst(e);
                    }
                }
                while (isOverflow()) {
                    HashEntry<K, V> first = lruQueue.getLast();
                    segment.remove(first.key, first.hash, null);
                    evicted.add(first);
                }
            } finally {
                accessQueue.clear();
            }
            return evicted;
        }

        private boolean isOverflow() {
            return lruQueue.size() > trimDownSize;
        }

        //@Override
        public void onEntryMiss(HashEntry<K, V> e) {
            lruQueue.addFirst(e);
        }

        /*
         * Invoked without holding a lock on Segment
         */
        //@Override
        public boolean onEntryHit(HashEntry<K, V> e) {
            accessQueue.add(e);
            return accessQueue.size() >= maxBatchQueueSize * batchThresholdFactor;
        }

        /*
         * Invoked without holding a lock on Segment
         */
        //@Override
        public boolean thresholdExpired() {
            return accessQueue.size() >= maxBatchQueueSize;
        }

        //@Override
        public void onEntryRemove(HashEntry<K, V> e) {
            assert lruQueue.remove(e);
            // we could have multiple instances of e in accessQueue; remove them all
            while (accessQueue.remove(e))
                ;
        }

        //@Override
        public void clear() {
            lruQueue.clear();
            accessQueue.clear();
        }

        //@Override
        public Eviction strategy() {
            return Eviction.LRU;
        }
    }

    static final class LIRS<K, V> implements EvictionPolicy<K, V> {
        private final static int MIN_HIR_SIZE = 2;
        private final Segment<K,V> segment;
        private final ConcurrentLinkedQueue<HashEntry<K, V>> accessQueue;
        private final LinkedHashMap<Integer, HashEntry<K, V>> stack;
        private final LinkedList<HashEntry<K, V>> queue;
        private final int maxBatchQueueSize;
        private final int lirSizeLimit;
        private final int hirSizeLimit;
        private int currentLIRSize;
        private final float batchThresholdFactor;

        public LIRS(Segment<K,V> s, int capacity, float lf, int maxBatchSize, float batchThresholdFactor) {
            this.segment = s;
            int tmpLirSize = (int) (capacity * 0.9);
            int tmpHirSizeLimit = capacity - tmpLirSize;
            if (tmpHirSizeLimit < MIN_HIR_SIZE) {
                hirSizeLimit = MIN_HIR_SIZE;
                lirSizeLimit = capacity - hirSizeLimit;
            } else {
                hirSizeLimit = tmpHirSizeLimit;
                lirSizeLimit = tmpLirSize;
            }
            this.maxBatchQueueSize = maxBatchSize > MAX_BATCH_SIZE ? MAX_BATCH_SIZE : maxBatchSize;
            this.batchThresholdFactor = batchThresholdFactor;
            this.accessQueue = new ConcurrentLinkedQueue<HashEntry<K, V>>();
            this.stack = new LinkedHashMap<Integer, HashEntry<K, V>>();
            this.queue = new LinkedList<HashEntry<K, V>>();
        }

        //@Override
        public Set<HashEntry<K, V>> execute() {
            Set<HashEntry<K, V>> evicted = new HashSet<HashEntry<K, V>>();
            try {
                for (HashEntry<K, V> e : accessQueue) {
                    if (present(e)) {
                        if (e.recency() == Recency.LIR_RESIDENT) {
                            handleLIRHit(e, evicted);
                        } else if (e.recency() == Recency.HIR_RESIDENT) {
                            handleHIRHit(e, evicted);
                        }
                    }
                }
                removeFromSegment(evicted);
            } finally {
                accessQueue.clear();
            }
            return evicted;
        }

        private void handleHIRHit(HashEntry<K, V> e, Set<HashEntry<K, V>> evicted) {
            boolean inStack = stack.containsKey(e.hashCode());
            if (inStack)
                stack.remove(e.hashCode());

            // first put on top of the stack
            stack.put(e.hashCode(), e);

            if (inStack) {
                assert queue.contains(e);
                queue.remove(e);
                e.transitionHIRResidentToLIRResident();
                switchBottomostLIRtoHIRAndPrune(evicted);
            } else {
                assert queue.contains(e);
                queue.remove(e);
                queue.addLast(e);
            }
        }

        private void handleLIRHit(HashEntry<K, V> e, Set<HashEntry<K, V>> evicted) {
            stack.remove(e.hashCode());
            stack.put(e.hashCode(), e);
            for (Iterator<HashEntry<K, V>> i = stack.values().iterator(); i.hasNext();) {
                HashEntry<K, V> next = i.next();
                if (next.recency() == Recency.LIR_RESIDENT) {
                    break;
                } else {
                    i.remove();
                    evicted.add(next);
                }
            }
        }

        private boolean present(HashEntry<K, V> e) {
            return stack.containsKey(e.hashCode()) || queue.contains(e);
        }

        //@Override
        public void onEntryMiss(HashEntry<K, V> e) {
            // initialization
            if (currentLIRSize + 1 < lirSizeLimit) {
                currentLIRSize++;
                e.transitionHIRResidentToLIRResident();
                stack.put(e.hashCode(), e);
            } else {
                if (queue.size() < hirSizeLimit) {
                    assert !queue.contains(e);
                    queue.addLast(e);
                } else {
                    boolean inStack = stack.containsKey(e.hashCode());

                    HashEntry<K, V> first = queue.removeFirst();
                    assert first.recency() == Recency.HIR_RESIDENT;
                    first.transitionHIRResidentToHIRNonResident();

                    stack.put(e.hashCode(), e);

                    if (inStack) {
                        e.transitionHIRResidentToLIRResident();
                        Set<HashEntry<K, V>> evicted = new HashSet<HashEntry<K, V>>();
                        switchBottomostLIRtoHIRAndPrune(evicted);
                        removeFromSegment(evicted);
                    } else {
                        assert !queue.contains(e);
                        queue.addLast(e);
                    }

                    // evict from segment
                    segment.remove(first.key, first.hash, null);
                }
            }
        }

        private void removeFromSegment(Set<HashEntry<K, V>> evicted) {
            for (HashEntry<K, V> e : evicted) {
                segment.remove(e.key, e.hash, null);
            }
        }

        private void switchBottomostLIRtoHIRAndPrune(Set<HashEntry<K, V>> evicted) {
            boolean seenFirstLIR = false;
            for (Iterator<HashEntry<K, V>> i = stack.values().iterator(); i.hasNext();) {
                HashEntry<K, V> next = i.next();
                if (next.recency() == Recency.LIR_RESIDENT) {
                    if (!seenFirstLIR) {
                        seenFirstLIR = true;
                        i.remove();
                        next.transitionLIRResidentToHIRResident();
                        assert !queue.contains(next);
                        queue.addLast(next);
                    } else {
                        break;
                    }
                } else {
                    i.remove();
                    evicted.add(next);
                }
            }
        }

        /*
         * Invoked without holding a lock on Segment
         */
        //@Override
        public boolean onEntryHit(HashEntry<K, V> e) {
            accessQueue.add(e);
            return accessQueue.size() >= maxBatchQueueSize * batchThresholdFactor;
        }

        /*
         * Invoked without holding a lock on Segment
         */
        //@Override
        public boolean thresholdExpired() {
            return accessQueue.size() >= maxBatchQueueSize;
        }

        //@Override
        public void onEntryRemove(HashEntry<K, V> e) {
            HashEntry<K, V> removed = stack.remove(e.hashCode());
            if (removed != null && removed.recency() == Recency.LIR_RESIDENT) {
                currentLIRSize--;
            }
            queue.remove(e);
            // we could have multiple instances of e in accessQueue; remove them all
            while (accessQueue.remove(e));
        }

        //@Override
        public void clear() {
            stack.clear();
            accessQueue.clear();
        }

        //@Override
        public Eviction strategy() {
            return Eviction.LIRS;
        }
    }

    /**
     * Segments are specialized versions of hash tables. This subclasses from ReentrantLock
     * opportunistically, just to simplify some locking and avoid separate construction.
     */
    static final class Segment<K, V> extends ReentrantLock implements Serializable {

        /*
         * Segments maintain a table of entry lists that are ALWAYS kept in a consistent state, so
         * can be read without locking. Next fields of nodes are immutable (final). All list
         * additions are performed at the front of each bin. This makes it easy to check changes,
         * and also fast to traverse. When nodes would otherwise be changed, new nodes are created
         * to replace them. This works well for hash tables since the bin lists tend to be short.
         * (The average length is less than two for the default load factor threshold.)
         * 
         * Read operations can thus proceed without locking, but rely on selected uses of volatiles
         * to ensure that completed write operations performed by other threads are noticed. For
         * most purposes, the "count" field, tracking the number of elements, serves as that
         * volatile variable ensuring visibility. This is convenient because this field needs to be
         * read in many read operations anyway:
         * 
         * - All (unsynchronized) read operations must first read the "count" field, and should not
         * look at table entries if it is 0.
         * 
         * - All (synchronized) write operations should write to the "count" field after
         * structurally changing any bin. The operations must not take any action that could even
         * momentarily cause a concurrent read operation to see inconsistent data. This is made
         * easier by the nature of the read operations in Map. For example, no operation can reveal
         * that the table has grown but the threshold has not yet been updated, so there are no
         * atomicity requirements for this with respect to reads.
         * 
         * As a guide, all critical volatile reads and writes to the count field are marked in code
         * comments.
         */

        private static final long serialVersionUID = 2249069246763182397L;

        /**
         * The number of elements in this segment's region.
         */
        transient volatile int count;

        /**
         * Number of updates that alter the size of the table. This is used during bulk-read methods
         * to make sure they see a consistent snapshot: If modCounts change during a traversal of
         * segments computing size or checking containsValue, then we might have an inconsistent
         * view of state so (usually) must retry.
         */
        transient int modCount;

        /**
         * The table is rehashed when its size exceeds this threshold. (The value of this field is
         * always <tt>(int)(capacity *
         * loadFactor)</tt>.)
         */
        transient int threshold;

        /**
         * The per-segment table.
         */
        transient volatile HashEntry<K, V>[] table;

        transient final EvictionPolicy<K, V> eviction;

        transient final EvictionListener<K, V> evictionListener;

        /**
         * The load factor for the hash table. Even though this value is same for all segments, it
         * is replicated to avoid needing links to outer object.
         * 
         * @serial
         */
        final float loadFactor;

        Segment(int cap, float lf, Eviction es, EvictionListener<K, V> listener) {
            loadFactor = lf;
            eviction = es.make(this, cap, lf);
            evictionListener = listener;
            setTable(HashEntry.<K, V> newArray(cap));
        }

        @SuppressWarnings("unchecked")
        static <K, V> Segment<K, V>[] newArray(int i) {
            return new Segment[i];
        }

        /**
         * Sets table to new HashEntry array. Call only while holding lock or in constructor.
         */
        void setTable(HashEntry<K, V>[] newTable) {
            threshold = (int) (newTable.length * loadFactor);
            table = newTable;
        }

        /**
         * Returns properly casted first entry of bin for given hash.
         */
        HashEntry<K, V> getFirst(int hash) {
            HashEntry<K, V>[] tab = table;
            return tab[hash & (tab.length - 1)];
        }

        /**
         * Reads value field of an entry under lock. Called if value field ever appears to be null.
         * This is possible only if a compiler happens to reorder a HashEntry initialization with
         * its table assignment, which is legal under memory model but is not known to ever occur.
         */
        V readValueUnderLock(HashEntry<K, V> e) {
            lock();
            try {
                return e.value;
            } finally {
                unlock();
            }
        }

        V get(Object key, int hash) {
            int c = count;
            if (c != 0) { // read-volatile
                V result = null;
                HashEntry<K, V> e = getFirst(hash);
                loop: while (e != null) {
                    if (e.hash == hash && key.equals(e.key)) {
                        V v = e.value;
                        if (v != null) {
                            result = v;
                            break loop;
                        } else {
                            result = readValueUnderLock(e); // recheck
                            break loop;
                        }
                    }
                    e = e.next;
                }
                // a hit
                if (result != null) {
                    if (eviction.onEntryHit(e)) {
                        Set<HashEntry<K, V>> evicted = attemptEviction(false);
                        // piggyback listener invocation on callers thread outside lock
                        if (evicted != null) {
                            for (HashEntry<K, V> he : evicted) {
                                evictionListener.evicted(he.key, he.value);
                            }
                        }
                    }
                }
                return result;
            }
            return null;
        }

        private Set<HashEntry<K, V>> attemptEviction(boolean lockedAlready) {
            Set<HashEntry<K, V>> evicted = null;
            boolean obtainedLock = !lockedAlready ? tryLock() : true;
            if (!obtainedLock && eviction.thresholdExpired()) {
                lock();
                obtainedLock = true;
            }
            if (obtainedLock) {
                try {
                    evicted = eviction.execute();
                } finally {
                    if (!lockedAlready)
                        unlock();
                }
            }
            return evicted;
        }

        boolean containsKey(Object key, int hash) {
            if (count != 0) { // read-volatile
                HashEntry<K, V> e = getFirst(hash);
                while (e != null) {
                    if (e.hash == hash && key.equals(e.key))
                        return true;
                    e = e.next;
                }
            }
            return false;
        }

        boolean containsValue(Object value) {
            if (count != 0) { // read-volatile
                HashEntry<K, V>[] tab = table;
                int len = tab.length;
                for (int i = 0; i < len; i++) {
                    for (HashEntry<K, V> e = tab[i]; e != null; e = e.next) {
                        V v = e.value;
                        if (v == null) // recheck
                            v = readValueUnderLock(e);
                        if (value.equals(v))
                            return true;
                    }
                }
            }
            return false;
        }

        boolean replace(K key, int hash, V oldValue, V newValue) {
            lock();
            Set<HashEntry<K, V>> evicted = null;
            try {
                HashEntry<K, V> e = getFirst(hash);
                while (e != null && (e.hash != hash || !key.equals(e.key)))
                    e = e.next;

                boolean replaced = false;
                if (e != null && oldValue.equals(e.value)) {
                    replaced = true;
                    e.value = newValue;
                    if (eviction.onEntryHit(e)) {
                        evicted = attemptEviction(true);
                    }
                }
                return replaced;
            } finally {
                unlock();
                // piggyback listener invocation on callers thread outside lock   
                if (evicted != null) {
                    for (HashEntry<K, V> he : evicted) {
                        evictionListener.evicted(he.key, he.value);
                    }
                }
            }
        }

        V replace(K key, int hash, V newValue) {
            lock();
            Set<HashEntry<K, V>> evicted = null;
            try {
                HashEntry<K, V> e = getFirst(hash);
                while (e != null && (e.hash != hash || !key.equals(e.key)))
                    e = e.next;

                V oldValue = null;
                if (e != null) {
                    oldValue = e.value;
                    e.value = newValue;
                    if (eviction.onEntryHit(e)) {
                        evicted = attemptEviction(true);
                    }
                }
                return oldValue;
            } finally {
                unlock();
                // piggyback listener invocation on callers thread outside lock
                if(evicted != null) {
                    for (HashEntry<K, V> he : evicted) {
                        evictionListener.evicted(he.key, he.value);
                    }                
                }
            }
        }

        V put(K key, int hash, V value, boolean onlyIfAbsent) {
            lock();
            Set<HashEntry<K, V>> evicted = null;
            try {
                int c = count;
                if (c++ > threshold && eviction.strategy() == Eviction.NONE) // ensure capacity
                    rehash();
                HashEntry<K, V>[] tab = table;
                int index = hash & (tab.length - 1);
                HashEntry<K, V> first = tab[index];
                HashEntry<K, V> e = first;
                while (e != null && (e.hash != hash || !key.equals(e.key)))
                    e = e.next;

                V oldValue;
                if (e != null) {
                    oldValue = e.value;
                    if (!onlyIfAbsent) {
                        e.value = value;
                        eviction.onEntryHit(e);
                    }
                } else {
                    oldValue = null;
                    ++modCount;
                    count = c; // write-volatile
                    if (eviction.strategy() != Eviction.NONE) {
                        if (c > tab.length) {
                            // remove entries;lower count
                            evicted = eviction.execute();
                            // re-read first
                            first = tab[index];
                        }
                        // add a new entry
                        tab[index] = new HashEntry<K, V>(key, hash, first, value);
                        // notify a miss
                        eviction.onEntryMiss(tab[index]);
                    } else {
                        tab[index] = new HashEntry<K, V>(key, hash, first, value);
                    }
                }
                return oldValue;
            } finally {
                unlock();
                // piggyback listener invocation on callers thread outside lock
                if(evicted != null) {
                    for (HashEntry<K, V> he : evicted) {
                        evictionListener.evicted(he.key, he.value);
                    }                
                }
            }
        }

        void rehash() {
            HashEntry<K, V>[] oldTable = table;
            int oldCapacity = oldTable.length;
            if (oldCapacity >= MAXIMUM_CAPACITY)
                return;

            /*
             * Reclassify nodes in each list to new Map. Because we are using power-of-two
             * expansion, the elements from each bin must either stay at same index, or move with a
             * power of two offset. We eliminate unnecessary node creation by catching cases where
             * old nodes can be reused because their next fields won't change. Statistically, at the
             * default threshold, only about one-sixth of them need cloning when a table doubles.
             * The nodes they replace will be garbage collectable as soon as they are no longer
             * referenced by any reader thread that may be in the midst of traversing table right
             * now.
             */

            HashEntry<K, V>[] newTable = HashEntry.newArray(oldCapacity << 1);
            threshold = (int) (newTable.length * loadFactor);
            int sizeMask = newTable.length - 1;
            for (int i = 0; i < oldCapacity; i++) {
                // We need to guarantee that any existing reads of old Map can
                // proceed. So we cannot yet null out each bin.
                HashEntry<K, V> e = oldTable[i];

                if (e != null) {
                    HashEntry<K, V> next = e.next;
                    int idx = e.hash & sizeMask;

                    // Single node on list
                    if (next == null)
                        newTable[idx] = e;

                    else {
                        // Reuse trailing consecutive sequence at same slot
                        HashEntry<K, V> lastRun = e;
                        int lastIdx = idx;
                        for (HashEntry<K, V> last = next; last != null; last = last.next) {
                            int k = last.hash & sizeMask;
                            if (k != lastIdx) {
                                lastIdx = k;
                                lastRun = last;
                            }
                        }
                        newTable[lastIdx] = lastRun;

                        // Clone all remaining nodes
                        for (HashEntry<K, V> p = e; p != lastRun; p = p.next) {
                            int k = p.hash & sizeMask;
                            HashEntry<K, V> n = newTable[k];
                            newTable[k] = new HashEntry<K, V>(p.key, p.hash, n, p.value);
                        }
                    }
                }
            }
            table = newTable;
        }

        /**
         * Remove; match on key only if value null, else match both.
         */
        V remove(Object key, int hash, Object value) {
            lock();
            try {
                int c = count - 1;
                HashEntry<K, V>[] tab = table;
                int index = hash & (tab.length - 1);
                HashEntry<K, V> first = tab[index];
                HashEntry<K, V> e = first;
                while (e != null && (e.hash != hash || !key.equals(e.key)))
                    e = e.next;

                V oldValue = null;
                if (e != null) {
                    V v = e.value;
                    if (value == null || value.equals(v)) {
                        oldValue = v;
                        // All entries following removed node can stay
                        // in list, but all preceding ones need to be
                        // cloned.
                        ++modCount;

                        // e was removed
                        eviction.onEntryRemove(e);

                        HashEntry<K, V> newFirst = e.next;
                        for (HashEntry<K, V> p = first; p != e; p = p.next) {
                            // allow p to be GC-ed
                            eviction.onEntryRemove(p);
                            newFirst = new HashEntry<K, V>(p.key, p.hash, newFirst, p.value);
                            // and notify eviction algorithm about new hash entries
                            eviction.onEntryMiss(newFirst);
                        }

                        tab[index] = newFirst;
                        count = c; // write-volatile
                    }
                }
                return oldValue;
            } finally {
                unlock();
            }
        }

        void clear() {
            if (count != 0) {
                lock();
                try {
                    HashEntry<K, V>[] tab = table;
                    for (int i = 0; i < tab.length; i++)
                        tab[i] = null;
                    ++modCount;
                    eviction.clear();
                    count = 0; // write-volatile
                } finally {
                    unlock();
                }
            }
        }
    }

    /* ---------------- Public operations -------------- */

    /**
     * Creates a new, empty map with the specified initial capacity, load factor and concurrency
     * level.
     * 
     * @param initialCapacity
     *            the initial capacity. The implementation performs internal sizing to accommodate
     *            this many elements.
     * @param loadFactor
     *            the load factor threshold, used to control resizing. Resizing may be performed
     *            when the average number of elements per bin exceeds this threshold.
     * @param concurrencyLevel
     *            the estimated number of concurrently updating threads. The implementation performs
     *            internal sizing to try to accommodate this many threads.
     * 
     * @param evictionStrategy
     *            the algorithm used to evict elements from this map
     * 
     * @param evictionListener
     *            the evicton listener callback to be notified about evicted elements
     * 
     * @throws IllegalArgumentException
     *             if the initial capacity is negative or the load factor or concurrencyLevel are
     *             nonpositive.
     */
    public BufferedConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel,
                    Eviction evictionStrategy, EvictionListener<K, V> evictionListener) {
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        
        if (evictionStrategy == null || evictionListener == null)
            throw new IllegalArgumentException();

        if (concurrencyLevel > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;

        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
        while (ssize < concurrencyLevel) {
            ++sshift;
            ssize <<= 1;
        }
        segmentShift = 32 - sshift;
        segmentMask = ssize - 1;
        this.segments = Segment.newArray(ssize);

        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
        int cap = 1;
        while (cap < c)
            cap <<= 1;
        
        for (int i = 0; i < this.segments.length; ++i)
            this.segments[i] = new Segment<K, V>(cap, loadFactor, evictionStrategy,
                            evictionListener);
    }

    public BufferedConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
        this(initialCapacity, loadFactor, concurrencyLevel, Eviction.LRU);
    }
    
    public BufferedConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel, Eviction evictionStrategy) {
        this(initialCapacity, loadFactor, concurrencyLevel, evictionStrategy, new NullEvictionListener<K, V>());
    }

    /**
     * Creates a new, empty map with the specified initial capacity and load factor and with the
     * default concurrencyLevel (16).
     * 
     * @param initialCapacity
     *            The implementation performs internal sizing to accommodate this many elements.
     * @param loadFactor
     *            the load factor threshold, used to control resizing. Resizing may be performed
     *            when the average number of elements per bin exceeds this threshold.
     * @throws IllegalArgumentException
     *             if the initial capacity of elements is negative or the load factor is nonpositive
     * 
     * @since 1.6
     */
    public BufferedConcurrentHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with the specified initial capacity, and with default load factor
     * (0.75) and concurrencyLevel (16).
     * 
     * @param initialCapacity
     *            the initial capacity. The implementation performs internal sizing to accommodate
     *            this many elements.
     * @throws IllegalArgumentException
     *             if the initial capacity of elements is negative.
     */
    public BufferedConcurrentHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with a default initial capacity (16), load factor (0.75) and
     * concurrencyLevel (16).
     */
    public BufferedConcurrentHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new map with the same mappings as the given map. The map is created with a capacity
     * of 1.5 times the number of mappings in the given map or 16 (whichever is greater), and a
     * default load factor (0.75) and concurrencyLevel (16).
     * 
     * @param m
     *            the map
     */
    public BufferedConcurrentHashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY),
                        DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
        putAll(m);
    }

    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     * 
     * @return <tt>true</tt> if this map contains no key-value mappings
     */
    public boolean isEmpty() {
        final Segment<K, V>[] segments = this.segments;
        /*
         * We keep track of per-segment modCounts to avoid ABA problems in which an element in one
         * segment was added and in another removed during traversal, in which case the table was
         * never actually empty at any point. Note the similar use of modCounts in the size() and
         * containsValue() methods, which are the only other methods also susceptible to ABA
         * problems.
         */
        int[] mc = new int[segments.length];
        int mcsum = 0;
        for (int i = 0; i < segments.length; ++i) {
            if (segments[i].count != 0)
                return false;
            else
                mcsum += mc[i] = segments[i].modCount;
        }
        // If mcsum happens to be zero, then we know we got a snapshot
        // before any modifications at all were made. This is
        // probably common enough to bother tracking.
        if (mcsum != 0) {
            for (int i = 0; i < segments.length; ++i) {
                if (segments[i].count != 0 || mc[i] != segments[i].modCount)
                    return false;
            }
        }
        return true;
    }

    /**
     * Returns the number of key-value mappings in this map. If the map contains more than
     * <tt>Integer.MAX_VALUE</tt> elements, returns <tt>Integer.MAX_VALUE</tt>.
     * 
     * @return the number of key-value mappings in this map
     */
    public int size() {
        final Segment<K, V>[] segments = this.segments;
        long sum = 0;
        long check = 0;
        int[] mc = new int[segments.length];
        // Try a few times to get accurate count. On failure due to
        // continuous async changes in table, resort to locking.
        for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) {
            check = 0;
            sum = 0;
            int mcsum = 0;
            for (int i = 0; i < segments.length; ++i) {
                sum += segments[i].count;
                mcsum += mc[i] = segments[i].modCount;
            }
            if (mcsum != 0) {
                for (int i = 0; i < segments.length; ++i) {
                    check += segments[i].count;
                    if (mc[i] != segments[i].modCount) {
                        check = -1; // force retry
                        break;
                    }
                }
            }
            if (check == sum)
                break;
        }
        if (true) { // Resort to locking all segments
            sum = 0;
            for (int i = 0; i < segments.length; ++i)
                segments[i].lock();
            for (int i = 0; i < segments.length; ++i)
                sum += segments[i].count;
            for (int i = 0; i < segments.length; ++i)
                segments[i].unlock();
        }
        if (sum > Integer.MAX_VALUE)
            return Integer.MAX_VALUE;
        else
            return (int) sum;
    }

    /**
     * Returns the value to which the specified key is mapped, or {@code null} if this map contains
     * no mapping for the key.
     * 
     * <p>
     * More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such
     * that {@code key.equals(k)}, then this method returns {@code v}; otherwise it returns {@code
     * null}. (There can be at most one such mapping.)
     * 
     * @throws NullPointerException
     *             if the specified key is null
     */
    public V get(Object key) {
        int hash = hash(key.hashCode());
        return segmentFor(hash).get(key, hash);
    }

    /**
     * Tests if the specified object is a key in this table.
     * 
     * @param key
     *            possible key
     * @return <tt>true</tt> if and only if the specified object is a key in this table, as
     *         determined by the <tt>equals</tt> method; <tt>false</tt> otherwise.
     * @throws NullPointerException
     *             if the specified key is null
     */
    public boolean containsKey(Object key) {
        int hash = hash(key.hashCode());
        return segmentFor(hash).containsKey(key, hash);
    }

    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the specified value. Note: This
     * method requires a full internal traversal of the hash table, and so is much slower than
     * method <tt>containsKey</tt>.
     * 
     * @param value
     *            value whose presence in this map is to be tested
     * @return <tt>true</tt> if this map maps one or more keys to the specified value
     * @throws NullPointerException
     *             if the specified value is null
     */
    public boolean containsValue(Object value) {
        if (value == null)
            throw new NullPointerException();

        // See explanation of modCount use above

        final Segment<K, V>[] segments = this.segments;
        int[] mc = new int[segments.length];

        // Try a few times without locking
        for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) {
            int sum = 0;
            int mcsum = 0;
            for (int i = 0; i < segments.length; ++i) {
                int c = segments[i].count;
                mcsum += mc[i] = segments[i].modCount;
                if (segments[i].containsValue(value))
                    return true;
            }
            boolean cleanSweep = true;
            if (mcsum != 0) {
                for (int i = 0; i < segments.length; ++i) {
                    int c = segments[i].count;
                    if (mc[i] != segments[i].modCount) {
                        cleanSweep = false;
                        break;
                    }
                }
            }
            if (cleanSweep)
                return false;
        }
        // Resort to locking all segments
        for (int i = 0; i < segments.length; ++i)
            segments[i].lock();
        boolean found = false;
        try {
            for (int i = 0; i < segments.length; ++i) {
                if (segments[i].containsValue(value)) {
                    found = true;
                    break;
                }
            }
        } finally {
            for (int i = 0; i < segments.length; ++i)
                segments[i].unlock();
        }
        return found;
    }

    /**
     * Legacy method testing if some key maps into the specified value in this table. This method is
     * identical in functionality to {@link #containsValue}, and exists solely to ensure full
     * compatibility with class {@link java.util.Hashtable}, which supported this method prior to
     * introduction of the Java Collections framework.
     * 
     * @param value
     *            a value to search for
     * @return <tt>true</tt> if and only if some key maps to the <tt>value</tt> argument in this
     *         table as determined by the <tt>equals</tt> method; <tt>false</tt> otherwise
     * @throws NullPointerException
     *             if the specified value is null
     */
    public boolean contains(Object value) {
        return containsValue(value);
    }

    /**
     * Maps the specified key to the specified value in this table. Neither the key nor the value
     * can be null.
     * 
     * <p>
     * The value can be retrieved by calling the <tt>get</tt> method with a key that is equal to the
     * original key.
     * 
     * @param key
     *            key with which the specified value is to be associated
     * @param value
     *            value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or <tt>null</tt> if there was no
     *         mapping for <tt>key</tt>
     * @throws NullPointerException
     *             if the specified key or value is null
     */
    public V put(K key, V value) {
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        return segmentFor(hash).put(key, hash, value, false);
    }

    /**
     * {@inheritDoc}
     * 
     * @return the previous value associated with the specified key, or <tt>null</tt> if there was
     *         no mapping for the key
     * @throws NullPointerException
     *             if the specified key or value is null
     */
    public V putIfAbsent(K key, V value) {
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        return segmentFor(hash).put(key, hash, value, true);
    }

    /**
     * Copies all of the mappings from the specified map to this one. These mappings replace any
     * mappings that this map had for any of the keys currently in the specified map.
     * 
     * @param m
     *            mappings to be stored in this map
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

    /**
     * Removes the key (and its corresponding value) from this map. This method does nothing if the
     * key is not in the map.
     * 
     * @param key
     *            the key that needs to be removed
     * @return the previous value associated with <tt>key</tt>, or <tt>null</tt> if there was no
     *         mapping for <tt>key</tt>
     * @throws NullPointerException
     *             if the specified key is null
     */
    public V remove(Object key) {
        int hash = hash(key.hashCode());
        return segmentFor(hash).remove(key, hash, null);
    }

    /**
     * {@inheritDoc}
     * 
     * @throws NullPointerException
     *             if the specified key is null
     */
    public boolean remove(Object key, Object value) {
        int hash = hash(key.hashCode());
        if (value == null)
            return false;
        return segmentFor(hash).remove(key, hash, value) != null;
    }

    /**
     * {@inheritDoc}
     * 
     * @throws NullPointerException
     *             if any of the arguments are null
     */
    public boolean replace(K key, V oldValue, V newValue) {
        if (oldValue == null || newValue == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        return segmentFor(hash).replace(key, hash, oldValue, newValue);
    }

    /**
     * {@inheritDoc}
     * 
     * @return the previous value associated with the specified key, or <tt>null</tt> if there was
     *         no mapping for the key
     * @throws NullPointerException
     *             if the specified key or value is null
     */
    public V replace(K key, V value) {
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        return segmentFor(hash).replace(key, hash, value);
    }

    /**
     * Removes all of the mappings from this map.
     */
    public void clear() {
        for (int i = 0; i < segments.length; ++i)
            segments[i].clear();
    }

    /**
     * Returns a {@link Set} view of the keys contained in this map. The set is backed by the map,
     * so changes to the map are reflected in the set, and vice-versa. The set supports element
     * removal, which removes the corresponding mapping from this map, via the
     * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and
     * <tt>clear</tt> operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
     * operations.
     * 
     * <p>
     * The view's <tt>iterator</tt> is a "weakly consistent" iterator that will never throw
     * {@link ConcurrentModificationException}, and guarantees to traverse elements as they existed
     * upon construction of the iterator, and may (but is not guaranteed to) reflect any
     * modifications subsequent to construction.
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

    /**
     * Returns a {@link Collection} view of the values contained in this map. The collection is
     * backed by the map, so changes to the map are reflected in the collection, and vice-versa. The
     * collection supports element removal, which removes the corresponding mapping from this map,
     * via the <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>, <tt>removeAll</tt>,
     * <tt>retainAll</tt>, and <tt>clear</tt> operations. It does not support the <tt>add</tt> or
     * <tt>addAll</tt> operations.
     * 
     * <p>
     * The view's <tt>iterator</tt> is a "weakly consistent" iterator that will never throw
     * {@link ConcurrentModificationException}, and guarantees to traverse elements as they existed
     * upon construction of the iterator, and may (but is not guaranteed to) reflect any
     * modifications subsequent to construction.
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

    /**
     * Returns a {@link Set} view of the mappings contained in this map. The set is backed by the
     * map, so changes to the map are reflected in the set, and vice-versa. The set supports element
     * removal, which removes the corresponding mapping from the map, via the
     * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and
     * <tt>clear</tt> operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
     * operations.
     * 
     * <p>
     * The view's <tt>iterator</tt> is a "weakly consistent" iterator that will never throw
     * {@link ConcurrentModificationException}, and guarantees to traverse elements as they existed
     * upon construction of the iterator, and may (but is not guaranteed to) reflect any
     * modifications subsequent to construction.
     */
    public Set<Map.Entry<K, V>> entrySet() {
        Set<Map.Entry<K, V>> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

    /**
     * Returns an enumeration of the keys in this table.
     * 
     * @return an enumeration of the keys in this table
     * @see #keySet()
     */
    public Enumeration<K> keys() {
        return new KeyIterator();
    }

    /**
     * Returns an enumeration of the values in this table.
     * 
     * @return an enumeration of the values in this table
     * @see #values()
     */
    public Enumeration<V> elements() {
        return new ValueIterator();
    }

    /* ---------------- Iterator Support -------------- */

    abstract class HashIterator {
        int nextSegmentIndex;
        int nextTableIndex;
        HashEntry<K, V>[] currentTable;
        HashEntry<K, V> nextEntry;
        HashEntry<K, V> lastReturned;

        HashIterator() {
            nextSegmentIndex = segments.length - 1;
            nextTableIndex = -1;
            advance();
        }

        public boolean hasMoreElements() {
            return hasNext();
        }

        final void advance() {
            if (nextEntry != null && (nextEntry = nextEntry.next) != null)
                return;

            while (nextTableIndex >= 0) {
                if ((nextEntry = currentTable[nextTableIndex--]) != null)
                    return;
            }

            while (nextSegmentIndex >= 0) {
                Segment<K, V> seg = segments[nextSegmentIndex--];
                if (seg.count != 0) {
                    currentTable = seg.table;
                    for (int j = currentTable.length - 1; j >= 0; --j) {
                        if ((nextEntry = currentTable[j]) != null) {
                            nextTableIndex = j - 1;
                            return;
                        }
                    }
                }
            }
        }

        public boolean hasNext() {
            return nextEntry != null;
        }

        HashEntry<K, V> nextEntry() {
            if (nextEntry == null)
                throw new NoSuchElementException();
            lastReturned = nextEntry;
            advance();
            return lastReturned;
        }

        public void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            BufferedConcurrentHashMap.this.remove(lastReturned.key);
            lastReturned = null;
        }
    }

    final class KeyIterator extends HashIterator implements Iterator<K>, Enumeration<K> {
        public K next() {
            return super.nextEntry().key;
        }

        public K nextElement() {
            return super.nextEntry().key;
        }
    }

    final class ValueIterator extends HashIterator implements Iterator<V>, Enumeration<V> {
        public V next() {
            return super.nextEntry().value;
        }

        public V nextElement() {
            return super.nextEntry().value;
        }
    }

    /**
     * Custom Entry class used by EntryIterator.next(), that relays setValue changes to the
     * underlying map.
     */
    final class WriteThroughEntry extends AbstractMap.SimpleEntry<K, V> {
        private static final long serialVersionUID = -1075078642155041669L;

        WriteThroughEntry(K k, V v) {
            super(k, v);
        }

        /**
         * Set our entry's value and write through to the map. The value to return is somewhat
         * arbitrary here. Since a WriteThroughEntry does not necessarily track asynchronous
         * changes, the most recent "previous" value could be different from what we return (or
         * could even have been removed in which case the put will re-establish). We do not and
         * cannot guarantee more.
         */
        public V setValue(V value) {
            if (value == null)
                throw new NullPointerException();
            V v = super.setValue(value);
            BufferedConcurrentHashMap.this.put(getKey(), value);
            return v;
        }
    }

    final class EntryIterator extends HashIterator implements Iterator<Entry<K, V>> {
        public Map.Entry<K, V> next() {
            HashEntry<K, V> e = super.nextEntry();
            return new WriteThroughEntry(e.key, e.value);
        }
    }

    final class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return new KeyIterator();
        }

        public int size() {
            return BufferedConcurrentHashMap.this.size();
        }

        public boolean contains(Object o) {
            return BufferedConcurrentHashMap.this.containsKey(o);
        }

        public boolean remove(Object o) {
            return BufferedConcurrentHashMap.this.remove(o) != null;
        }

        public void clear() {
            BufferedConcurrentHashMap.this.clear();
        }
    }

    final class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator();
        }

        public int size() {
            return BufferedConcurrentHashMap.this.size();
        }

        public boolean contains(Object o) {
            return BufferedConcurrentHashMap.this.containsValue(o);
        }

        public void clear() {
            BufferedConcurrentHashMap.this.clear();
        }
    }

    final class EntrySet extends AbstractSet<Map.Entry<K, V>> {
        public Iterator<Map.Entry<K, V>> iterator() {
            return new EntryIterator();
        }

        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            V v = BufferedConcurrentHashMap.this.get(e.getKey());
            return v != null && v.equals(e.getValue());
        }

        public boolean remove(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            return BufferedConcurrentHashMap.this.remove(e.getKey(), e.getValue());
        }

        public int size() {
            return BufferedConcurrentHashMap.this.size();
        }

        public void clear() {
            BufferedConcurrentHashMap.this.clear();
        }
    }

    /* ---------------- Serialization Support -------------- */

    /**
     * Save the state of the <tt>ConcurrentHashMap</tt> instance to a stream (i.e., serialize it).
     * 
     * @param s
     *            the stream
     * @serialData the key (Object) and value (Object) for each key-value mapping, followed by a
     *             null pair. The key-value mappings are emitted in no particular order.
     */
    private void writeObject(java.io.ObjectOutputStream s) throws IOException {
        s.defaultWriteObject();

        for (int k = 0; k < segments.length; ++k) {
            Segment<K, V> seg = segments[k];
            seg.lock();
            try {
                HashEntry<K, V>[] tab = seg.table;
                for (int i = 0; i < tab.length; ++i) {
                    for (HashEntry<K, V> e = tab[i]; e != null; e = e.next) {
                        s.writeObject(e.key);
                        s.writeObject(e.value);
                    }
                }
            } finally {
                seg.unlock();
            }
        }
        s.writeObject(null);
        s.writeObject(null);
    }

    /**
     * Reconstitute the <tt>ConcurrentHashMap</tt> instance from a stream (i.e., deserialize it).
     * 
     * @param s
     *            the stream
     */
    private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException {
        s.defaultReadObject();

        // Initialize each segment to be minimally sized, and let grow.
        for (int i = 0; i < segments.length; ++i) {
            segments[i].setTable(new HashEntry[1]);
        }

        // Read the keys and values, and put the mappings in the table
        for (;;) {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            if (key == null)
                break;
            put(key, value);
        }
    }
}
